Search for heavy Higgs bosons produced in association with a pair of ttbar in one or two opposite-sign leptons final state using the full Run 2 proton-proton collisions at sqrt(s)=13 TeV with the ATLAS detector

Abstract

The Large Hadron Collider (LHC) is an accelerator that provides a high energy condition to generate fundamental particles. Studying one of the fundamental particles, Higgs boson, is the target of this thesis. The Higgs boson (h) predicted by the Standard Model (SM) with a mass of 125 GeV was discovered at the LHC experiment in 2012. The beyond SM, type-II two-Higgs-doublet model (2HDM) predicts the existence of two opposite parity charged but electrically neutral Higgs bosons (H/A), which are heavier than h. As part of this thesis, the production of the heavy Higgs boson and a pair of top quarks, with the former decaying into a pair of top quarks, ttH/A->tttt, in the final state of one or two opposite charged leptons is studied. The dataset used in this thesis is collected from an integrated luminosity of 139 fb^(-1) proton-proton collisions at a center-of-mass energy of 13 TeV by the ATLAS detector in the LHC. A multivariate classifier, Artificial Neural Network (NN), is used to improve one of the SM processes, tt+jets, modelling. An other multivariate classifier, Boosted Decision Trees (BDT), is used to separate the signal from the background. The 95% CL expected upper limits on the cross-section times branching ratio of H/A->tt for the heavy Higgs boson with mass of 400 GeV to 1000 GeV are estimated by certain statistical techniques.